Research for putting a magnetic random access memory (MRAM) using the magnetoresistive effect (TMR: Tunneling Magneto Resistive) into practical use is being extensively made all over the world. Among other MRAMs, a spin transfer torque MRAM using the physics of spin transfer torque magnetization reversal is expected as a technique that will realize a large-scale non-volatile random access memory.
Unfortunately, the spin transfer torque MRAM has several serious technical problems. One problem is the processing of a magnetic tunnel junction film (MTJ film) having a multilayered film including a metal and metal oxide.
An MTJ film processing method according to a first prior art will be explained below. In this method, a lower electrode layer, MTJ film, and upper electrode layer are sequentially stacked after a lower contact layer to be connected to a switching element is formed. After that, a photoresist mask is formed on the upper electrode layer, and a photoresist pattern is transferred onto the upper electrode layer by etching. Then, the MTJ film is processed using etching. Since this processing of the MTJ film uses a method including physical etching, it is difficult to increase the etching selectivity to the material of, e.g., a lower silicon oxide film (an interlayer dielectric film around the lower contact layer), so there is the possibility that the lower layer is largely etched during overetching. When processing the MTJ film, therefore, the lower electrode layer is normally used as an etching stopper layer. After that, it is necessary to separately process the lower electrode layer again by using lithography and dry etching.
An MTJ film processing method according to a second prior art will now be explained. In this method, a pattern is transferred onto an upper electrode layer after a photoresist mask is formed in the same manner as in the above-mentioned first prior art. Then, an MTJ film and lower electrode layer are simultaneously etched by using the upper electrode layer. This etching is stopped when a lower interlayer dielectric film is exposed.
The problem of the first prior art is as follows. In the first prior art, the MTJ film and lower electrode layer are processed by using different lithography techniques. This makes it necessary to give a design margin between the MTJ and lower electrode layer, and relax the design of the whole cell to some extent. Accordingly, the cell is difficult to downsize.
The problems of the second prior art are as follows. In the second prior art, the MTJ film and lower electrode layer are simultaneously processed without using the lower electrode layer as an etching stopper layer. This processing of the MTJ film includes physical etching. Therefore, the lower interlayer dielectric film is largely etched back during overetching after the interlayer dielectric film is exposed. To prevent this, the lower interlayer dielectric film must be thickened. This interferes with the downsizing of the lower contact layer. In addition, when forming the MTJ and lower contact layer by close-packed design, the lower contact layer exposed by misalignment is etched away to a certain degree. Consequently, the reliability of the contact decreases. Giving a design margin between the lower contact layer and MTJ in order to avoid this problem also interferes with the downsizing of the cell.
As explained in the problems of the above-mentioned first and second prior arts, it is difficult for the conventional MTJ processing techniques to downsize a cell.